Field of the Invention
The present invention relates to a vehicle body structure, especially relates to a vehicle lower body frame structure which supports a lower part of a vehicle cabin.
Description of the Related Art
A vehicle has a lower part of the vehicle cabin, where occupants are present, supported by a vehicle lower body frame structure.
The vehicle cabin of most vehicles has a backbone at the center in the vehicle width direction, and front seats are disposed in a lower part on both sides in the vehicle width direction across the backbone. Rear seats are disposed in a lower part of the vehicle cabin, on the rear side of the front seats. Therefore, the vehicle lower body frame structure employs a lattice-shaped structure, as disclosed in Japanese Patent Laid-Open No. 2007-118736, which has a pair of side members disposed under the vehicle cabin on both sides in the vehicle width direction and extending along the vehicle front-rear direction, a front seat cross member disposed between these side members and passed under the front seats to couple together the side members, and a rear seat cross member passed under the rear seats to couple together the side members.
To secure the crash safety, the vehicle cabin where occupants are present is required to have the rigidity and strength to withstand impact load.
However, in the event of a frontal offset crash, i.e., a frontal crash in which impact load is applied intensively to one side of the vehicle front, the vehicle lower body frame structure of a simple lattice shape as described above cannot easily disperse the impact load to the front seat cross member and the rear seat cross member. As a result, the impact load is likely to be input intensively to only the side member on that one side. Accordingly, the side member on the crash side is likely to move toward the vehicle rear side, which may result in distortion of the vehicle lower body frame structure and deformation of the vehicle cabin.
In the case of a side crash of a vehicle, especially a pole crash, i.e., a crash in which a vehicle crashes into a pole and impact load is applied locally to the vehicle side, as the impact load is applied intensively to one part of the vehicle side, the front cross member and the rear cross member disposed under the front seats and the rear seats fail to support the load, so that only the vehicle side to which the impact load is applied deforms significantly toward the vehicle inner side. This may result in distortion of the vehicle lower body frame structure and deformation of the vehicle cabin.
Moreover, when impact is applied to the seatbelt of the front seat or the rear seat in a frontal crash of a vehicle, the lattice-shaped vehicle lower body frame structure causes the impact load to be applied intensively to only the front seat cross member or the rear seat cross member through the seatbelt anchor. Accordingly, an excessive bending stress is exerted on the front seat cross member, which may result in deformation of the front seat cross member.
To suppress deformation of the vehicle cabin due to deformation of the side members or the cross members, reinforcement measures, such as increasing the thickness or the cross-sectional area of the side members and the cross members or providing a reinforcing member, have been conventionally taken.
However, impact load tends to become excessive. Therefore, the measure of simply reinforcing the individual members cannot sufficiently cope with such impact load and can hardly suppress deformation of the vehicle cabin.